The basic oxygen steelmaking process is as follows. Molten pig iron (referred to as “hot metal”) from a blast furnace is poured into a large refractory-lined container called a ladle. The metal in the ladle is either sent directly for basic oxygen steelmaking or diverted first to a pretreatment stage. Pretreatment of the blast furnace hot metal is used to reduce the content of sulfur, silicon, and phosphorus. In the desulfurizing pretreatment, a lance, sometimes called a dip lance, is lowered into the molten iron in the ladle and several hundred kilograms of powdered magnesium (or sometimes other materials, such as lime, calcium carbide or combinations thereof which can be added separately through different lances) are added by pneumatic injection, preferably with an inert gas. Sulfur impurities are reduced to magnesium sulfide in a violent exothermic reaction. The magnesium sulfide is then raked off. Similar pretreatment is possible for desiliconization and dephosphorization using mill scale (iron oxide) and lime as reagents. The decision to pretreat depends on the quality of the blast furnace hot metal and the required final quality of the basic oxygen furnace (“BOF”) steel.
Filling the furnace with the ingredients is called charging. The BOF process is autogenous, the required thermal energy is produced during the process. Maintaining the proper charge balance, the ratio of hot metal to scrap, is therefore very important. The BOF vessel may be one-fifth filled with steel scrap. Molten iron from the ladle, whether or not treated as described above, is added as required by the charge balance. A typical chemistry of hot metal charged into the BOF vessel is: 4% C, 0.2-0.8% Si, 0.08%-0.18% P, and 0.01-0.04%, or, more narrowly, 0.002-0.005% S.
The BOF vessel is then set upright and a water-cooled lance is lowered down into it. The lance blows 99% pure oxygen onto the steel and iron, igniting the carbon dissolved in the steel and burning it to form carbon monoxide and carbon dioxide, causing the temperature to rise to about 1700° C. This melts the scrap, lowers the carbon content of the molten iron and helps remove unwanted chemical elements. It is this use of oxygen instead of air that improves upon the now discontinued Bessemer Process, as the nitrogen (and other gases) in air do not react with the charge as oxygen does. High purity oxygen is blown into the furnace or BOF vessel through a vertically oriented water-cooled lance with velocities faster than Mach 1.
Details and parameters of the above description may vary; it is intended to provide the reader with a typical background for the invention.
Fluxes (burnt lime or dolomite) are fed into the BOF vessel to form slag, which absorbs impurities of the steelmaking process. A prior art process fed a crushed burnt lime (80 mm×down) flux to help form the slag. During blowing the metal in the vessel forms an emulsion with the slag, facilitating the refining process. Near the end of the blowing cycle, which takes about 20 minutes, the temperature is measured and samples are taken. The samples are tested and a computer analysis of the steel given within six minutes. A typical chemistry of the blown metal is 0.03-0.06% C, 0.05-0.1% Mn, 0.01-0.03% Si, 0.01-0.03% S and P.
The lime flux removes impurities and forms a slag that can be separated from the steel and poured from the furnace as a liquid. It also reduces refractory wear and gunning. Soft burnt flux lime is used, unless a finer product is required by specialty furnace injection applications. A BOF process commonly uses the soft burnt flux lime sized at 80 mm×down. The 80×down sized lime created several costly and time consuming problems at the facility. The excess fines drawn off by the dust collection system was material that was lost. The dust collection fines not only represented a significant loss of raw material, but also created a waste disposal stream having an unacceptably high pH, which required expensive treatment. In addition, the fines from the 80 mm×down sized material seriously overloaded the bag house and wet scrubbers. The facility was faced with serious environmental and maintenance problems along with the loss of lime during the lime charging process and all the associated costs incurred.
Typically, from 8-15%, or even occasionally as high as 20%, by weight of the soft-burnt (80 mm×down) flux lime fed to the basic oxygen furnace in the above mentioned commercial process was lost in the dust collection system prior to the present invention. The invention not only recovers this portion of the original 80 mm×down feed (or other defined lime fines) and obviates the costs associated with dogging and waste disposal, but also creates a high-value product from it.